Effective, efficient systemic circulation depends on proper cardiac function. Proper cardiac function, in turn, relies on the synchronized contractions of the heart at regular intervals. When normal cardiac rhythm is initiated by the sinoatrial node, the heart is said to be in sinus rhythm. However, when the heart experiences irregularities in its coordinated contraction, due to electrophysiologic abnormalities that are either inherited, induced, or caused by disease, the heart is denoted to be arrhythmic. The resulting cardiac arrhythmia impairs cardiac efficiency and can be a potential life threatening event.
In a heart monitoring system it is often desirable to distinguish between ventricular complexes that are conducted by the intrinsic conduction system from the atria, and ventricular complexes that originate in the ventricle. Cardiac arrhythmias arising from the atria of the heart are called supraventricular tachyarrhythmias (SVTs). Cardiac arrhythmias arising from the ventricular region of the heart are called ventricular tachyarrhythmias (VTs). SVTs and VTs are morphologically and physiologically distinct events. VTs take many forms, including ventricular fibrillation and ventricular tachycardia. Ventricular fibrillation is a condition denoted by extremely rapid, nonsynchronous, and ineffective contractions of the ventricles where the ventricular complexes of ventricular fibrillation arise from multiple locations. This condition is fatal unless the heart is returned to sinus rhythm within a few minutes. Ventricular tachycardia are conditions denoted by a rapid heart beat in excess of 120 beats per minute, but frequently as high as 150 to 350 beats per minute, that has its origin in a single location within the ventricle. This location, which is frequently abnormal cardiac tissue, typically results from damage to the ventricular myocardium from a myocardial infarction or some other heart muscle disease process. Ventricular tachycardia can and frequently does degenerate into ventricular fibrillation.
SVTs also take many forms, including atrial fibrillation, sinus tachycardia and atrial flutter. These conditions are characterized by rapid contractions of the atria. Besides being hemodynamically inefficient, the rapid contractions of the atria can also result in an elevated ventricular rate. This occurs when the aberrant electrical impulse in the atria are transmitted to the ventricles via the intrinsic conduction system. Although an SVT can result in significant symptoms for the patient, it is usually not life threatening.
Transvenous implantable cardioverter/defibrillators (transvenous ICDs) have been established as an effective treatment for patients with serious ventricular tachyarrhythmias. Transvenous ICDs are able to recognize and treat tachyarrhythmias with a variety of therapies. These therapies range from providing anti-tachycardia pacing or cardioversion energy for treating ventricular tachycardia to high energy shock for treating ventricular fibrillation. Usually, the transvenous ICD delivers these therapies in sequence starting with anti-tachycardia pacing and then proceeding to cardioversion (or low) energy and then, finally, high energy shocks. Sometimes only one of these is selected depending upon the tachyarrhythmia detected. This sequence or selection of therapy is called “tiered” therapy. To effectively deliver these treatments, the ICD must first classify the type of tachyarrhythmia that is occurring, after which appropriate therapy is provided to the heart. A problem arises, however, when the ICD delivers therapy to what was mistakenly classified as a ventricular tachycardia, but was actually a high ventricular rate caused and sustained by an SVT.
A major limitation of both past and present transvenous ICDs is inaccuracy in differentiating tachycardias requiring therapy, and tachycardias for which therapy is not appropriate. Inappropriate electrical therapy from currently available commercial and investigational devices has been reported during documented periods of sinus rhythm, sinus tachycardia and supraventricular tachycardias including atrial flutter and atrial fibrillation.
Besides being painful, when a transvenous ICD delivers inappropriate treatment to a patient, it is extremely disconcerting to the patient. Moreover, it can induce worse cardiac arrhythmias and even lead to a deterioration in cardiac contraction strength. Accurate discrimination of an SVT versus a potentially lethal ventricular tachycardia is, therefore, an important factor in ensuring that appropriate therapy is delivered to an arrhythmic heart.
For the reasons stated above, and for other reasons stated below, which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for providing a reliable system to discriminate between SVT and ventricular tachycardia and SVT and ventricular fibrillation.